(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD). More particularly, the present invention relates to an LCD having a modified electrode array.
(b) Description of the Related Art
Generally, an LCD is a display having two substrates and a liquid crystal layer therebetween. A plurality of electrodes are formed on the inner surfaces of one or both of the substrates. A pair of polarizers are attached to the outer surfaces of the substrates, and the liquid crystal layer serves as an optical switch. When a potential difference is applied to the electrodes, liquid crystal molecules are re-arranged according to the potential difference. The re-arranged liquid crystal molecules scatter the incident light that has passed through first polarizers, and change the transmission characteristics of the light, thereby controlling the transmittance of the light through second polarizers (analyzer) and displaying images.
As an example of a conventional LCD, U.S. Pat. No. 5,576,861 discloses a twisted nematic LCD (TN-LCD) where an upper electrode and a lower electrode are respectively formed on the inner surfaces of upper and lower substrates and a nematic liquid crystal material is injected therebetween. The liquid crystal molecules are twisted parallel to the substrates. The potential difference applied between the two yields an electric field perpendicular to the substrates. The liquid crystal molecules are re-arranged such that torques due to a dielectric anisotropy and an aligning treatment is balanced against each other. The torque due to the dielectric anisotropy forces the long axes of the liquid crystal molecules to be parallel to the field direction, and the magnitude of this torque depends on the intensity of the electric field. The elastic torque generated by the aligning treatment like a rubbing forces the long axes of the liquid crystal molecules to be parallel to a predetermined direction. When the direction of the liquid crystal twists by 90 degrees on going from the lower electrode to the upper electrode, and the polarization directions of the polarizers are perpendicular to each other, the polarization of the incident light, in absence of the electric field, rotates by 90 degrees. Thus, the light passes through the analyzer, thereby causing a white state. However, when sufficient electric field is applied to the liquid crystal layer, the incident light passes through the liquid crystal layer without changing its polarization. Consequently, the light cannot pass through the analyzer, thereby causing black state.
As another example of a conventional LCD, U.S. Pat. No. 5,598,285 discloses an LCD, where two linear electrodes parallel to each other are formed on either of the two substrates. A liquid crystal layer lies over the region between the two electrodes, and where the liquid crystal molecules are aligned parallel to the substrates. The potential difference between the two electrodes yields an electric field substantially parallel to the substrates and perpendicular to the two electrodes. The liquid crystal molecules are re-arranged such that the torque due to the dielectric anisotropy and the elastical torque due to rubbing are balanced against each other. When the polarization directions of the polarizers are perpendicular to each other, in absence of electric field, the crossed polarizer blocks the incident light and makes the liquid crystal display to be in a black state. However, when sufficient electric field is applied to the liquid crystal layer, the polarization of the incident light varies and the light passes through the analyzer, thereby causing a white state.
The above-mentioned LCDs have disadvantages described hereinafter respectively.
The principal disadvantage of the TN-LCD is its narrow viewing angle. In the TN-LCD, the larger an angle made by the direction of the user's eye and the direction normal to a surface of a display, the larger the value Δnd where birefringence Δn is the difference of the refractive indices between the directions of the long axes and the short axes of the liquid crystal molecules and d is the thickness of the liquid crystal layer. Accordingly, the contrast, which is defined as the luminance of the brightest state divided by that of the darkest state, decreases dramatically. In addition, gray inversion phenomenon also occurs. Accordingly, the viewing angle that provides the contrast of 10 is very narrow, and thus image quality is deteriorated when viewed at an angle greater than the viewing angle.
To compensate the viewing angle, methods using phase difference compensating films are suggested in U.S. Pat. No. 5,576,861, but they have disadvantages in manufacturing cost and the number of the process steps since the phase difference compensating films are additionally attached. Furthermore, the satisfactory viewing angle may not be still obtained even though the phase retardation compensation films are used.
The U.S. Pat. No. 5,598,285 also has disadvantages in power consumption and aperture ratio. The LCD disclosed in the U.S. Pat. No. 5,598,285 has an electric field of which strength is dependent on the positions. The field strength becomes weaker as it moves further away from the electrodes. Therefore, in order to obtain sufficient field strength at the point far from the electrodes, high driving voltage is required. In addition, since all the electrodes are formed on one substrate and storage capacitors are formed to obtain sufficient capacitance, the aperture ratio is small.
In the meantime, since the liquid crystal display is a passive display, it requires an external light source. A white light is usually used for the light source of the liquid crystal display, and red, green and blue color filters are used for color display. The color filters are formed on one of the substrates, and a black matrix for preventing light leakage at the boundaries of the color filters is formed therebetween.
The light from the light source changes its properties, such as polarization, in the liquid crystal layer, and the transmittance of the light depends on the wavelength of the light. The transmittance also depends on the driving mode of the liquid crystal display.
In the case of TN LCDs, the transmittance of the blue light differs from those of the red and green lights by 10%. Moreover, the IPS LCD has the difference of the transmittances of the blue, red and green lights more than 40%.
In order to reduce the difference in the transmittance, two methods are conventionally used. One is using a backlight unit and a driving circuit and the other is making a cell gap to be different for the pixels of different colors by adjusting the height of the color filters. However, the former method may increase the manufacturing cost and the number of process steps, and the latter may cause uneven rubbing.